1. Field of the Invention
This invention relates to a wiper apparatus used for wiping raindrops away from a surface of a windshield of, for example, an automobile with a wiper blade connected with a wiper motor and, more particularly, to a wiper apparatus which works automatically by sensing moisture of raindrops or the like, and amount of rainfall.
2. Description of the Prior Art
Heretofore, in regard to a wiper apparatus which works automatically by sensing raindrops, there has been the wiper apparatus disclosed in Japanese Patent Disclosure (Kokai) No. 59-140146/1984, for example. FIG. 10 and FIG. 11 show the wiper apparatus disclosed in the above-mentioned document.
In a wiper apparatus shown in the Figures, a raindrop detector 103 which is provided with a vibration sensor 101 comprising a piezo-electric device, an electrostriction device or the like and an amplifier circuit 102 for amplifying a signal produced from vibration sensor 101, is disposed on a hood 111a of a vehicle body 111 so as to receive raindrops. Vibration caused by collision of the raindrops is sensed and transformed into an electric signal by the vibration sensor 101 at a rainfall. The electric signal is amplified by said amplifier circuit 102 provided in said rain drop detector 103 and processed in a control circuit 104, and wiper motor 107 is actuated by supplying an electric current from a power source 105 through a driving circuit 106. And a wiper blade 109 connected with the wiper motor 107 through a linkage 108 oscillates reciprocatingly on a wiped surface 110a of a windshield glass 110 so as to wipe the wiped surface 110a.
However, in the abovementioned conventional wiper apparatus 110, the vibration caused by the collision of the raindrops is simply transformed into the electric signal through the vibration sensor 101 by using merely the raindrop detector 103, accordingly, there is the possibility that the wiper apparatus 100 works by sensing the vibration caused by objects other than raindrops such as pebbles, sand, dust or the like while traveling or also vibration caused by closing the door. In the case of decreasing the sensitivity of the vibration sensor 101 in order to avoid the aforementioned trouble, there is a problem in that the wiper apparatus 100 sometimes does not work under the small rainfall such as a drizzling rain.
Furthermore, it is necessary to actuate the wiper motor 7 by manual operation when vapor in the air condenses on the windshield glass 10, or the windshield glass is doused with water splashed by a car running in the opposite direction while traveling, for example because there is no vibration to be sensed by the vibration sensor 101. Therefore, there is another problem since the manual operation is troublesome in such a case.